This invention relates to producing a high emittance, low absorptance multilayer coating on aluminum or its alloys, and is particularly concerned with the provision of an inorganic coating on anodized aluminum or its alloys, having a low absorptance with controlled high emissivity and a minimum coating weight, and with procedure for achieving such a multilayer coating.
In space, there is no atmosphere to conduct heat to or from a spacecraft. Therefore, all heat gain or loss must be by radiation. Radiation is accomplished through the use of thermal control surfaces which can absorb solar radiation and emit radiation to space. These surfaces have a range of desirable values for solar absorptivity (.alpha.) and infrared emissivity (.epsilon.). For surfaces such as the radiators, it is important to absorb as little solar radiation as possible (low .alpha.) while radiating as much heat as possible to space (high .epsilon.).
The .alpha. and .epsilon. properties of the thermal control surfaces must be stable to maintain the temperatures of the spacecraft in the range required for effective operation. However, spacecraft which are in orbit near the earth (commonly called the low earth orbit or LEO) experience a hostile space environment consisting of atomic oxygen, ultraviolet radiation, charged particles, and contamination from other spacecraft components. These factors have been known to degrade the optical properties of spacecraft thermal control surfaces.
The development of a suitable long-life thermal control coating particularly on aluminum or its alloys, is therefore essential for the longevity of spacecraft structures. This coating must also be economical and easy to handle and apply to structures. Common radiator coatings include organic and inorganic white paints, silver-coated TEFLON films, and silver-coated quartz tiles and anodic coatings. Although organic coatings can provide the desired optical properties, such as silicone and fluorocarbon base coatings, they are attacked and erode in the LEO environment. Inorganic paints can achieve high emissivity but when applied to aluminum or alloys thereof, the substantial thickness of such paint coating causes it to weigh more than anodic aluminum coatings. In addition, the application of an inorganic paint to a bare aluminum surface provides substantially no corrosion protection and consequently requires strict environmental control from time of manufacture to flight into space. TEFLON is not resistant to the LEO environment. Quartz tiles have been very labor intensive to install particularly for the complex geometry of most spacecraft and are quite fragile. Anodic coatings of aluminum are one of the most attractive thermal coating systems because of the thinness and hence light weight of the anodic coating, it is integral with the aluminum substrate, it does not spall or chip even from micrometeoroid/debris impact, it provides corrosion protection terrestrially, and is completely resistant to erosion from atomic oxygen. In addition, relatively high emissivities can be obtained.
Anodizing is an electrolytic process that produces an oxide film on the surface of a metal. When aluminum is anodized in a sulfuric acid electrolyte, a porous film of aluminum oxide is formed on the surface of the part. Anodized 5657 aluminum represents a promising candidate for the thermal control coating of the radiators. While anodized aluminum surfaces have high emissivities, the drawbacks associated with anodic coatings is the undesirably high solar absorptance obtained with some anodized aluminum alloys and the increase in solar absorptance that occurs with LEO space exposure.
Representative of the prior art is U.S. Pat. No. 4,526,671, directed to the coloration of aluminum and its alloys for use as decorative materials and the like. According to the patent a white or grayish white substance can be formed by dipping, in the first step, aluminum or an aluminum alloy having an anodically oxidized film in a first solution containing a specific salt or electrolyzing with the first solution, thereby causing the product from this salt to enter into the micropore of said film and then, in the subsequent second step, dipping the product from the first step in a second solution containing a substance which reacts with the product from the salt to be converted into a white or grayish white compound or electrolyzing with the second solution.
U.S. Pat. No. 4,111,851 discloses a coating characterized by low thermal absorption, high thermal emittance and high electrical conductivity comprising (a) a fired oxide pigment comprising a minor amount of aluminum oxide and a major amount of zinc oxide; (b) a vehicle-binder comprising an alkali metal silicate; and (c) sufficient water to provide a mixture suitable for application to a substrate. The fired oxide pigment may further include a minor amount of cobalt oxide. The resulting coating is particularly useful for coating the surfaces of spacecraft and similar objects. The patent states that aluminum can be used as a substrate but that anodized aluminum should be avoided.
U.S. Pat. No. 4,397,716 discloses anodizing aluminum surfaces in chromic acid as the anodizing electrolyte to obtain an anodized coating adapted to be exposed to solar radiation and having a thermal emittance in the range of 0.10 to 0.72 and a solar absorptance in the range of 0.2 to 0.4. However a higher thermal emittance is required for more efficient spacecraft thermal surfaces, as noted above.
One object of the invention is to provide a high emittance low absorptance coating on aluminum or its alloys.
Another object is the provision of a multilayer coating on aluminum or its alloys having low absorptance with controlled high emissivity and a minimum coating weight.
A still further object is to provide a coating having the above characteristics on an anodized aluminum substrate surface, and which provides corrosion protection without special environmental controls.
Yet another object is the provision of procedure for achieving the above coatings.